On Wed, 10 Nov 2021 12:32:53 GMT, Alan Hayward <d...@openjdk.java.net> wrote:
> PAC is an optional feature in AArch64 8.3 and is compulsory in v9. One > of its uses is to protect against ROP based attacks. This is done by > signing the Link Register whenever it is stored on the stack, and > authenticating the value when it is loaded back from the stack. If an > attacker were to try to change control flow by editing the stack then > the authentication check of the Link Register will fail, causing a > segfault when the function returns. > > On a system with PAC enabled, it is expected that all applications will > be compiled with ROP protection. Fedora 33 and upwards already provide > this. By compiling for ARMv8.0, GCC and LLVM will only use the set of > PAC instructions that exist in the NOP space - on hardware without PAC, > these instructions act as NOPs, allowing backward compatibility for > negligible performance cost (2 NOPs per non-leaf function). > > Hardware is currently limited to the Apple M1 MacBooks. All testing has > been done within a Fedora Docker image. A run of SpecJVM showed no > difference to that of noise - which was surprising. > > The most important part of this patch is simply compiling using branch > protection provided by GCC/LLVM. This protects all C++ code from being > used in ROP attacks, removing all static ROP gadgets from use. > > The remainder of the patch adds ROP protection to runtime generated > code, in both stubs and compiled Java code. Attacks here are much harder > as ROP gadgets must be found dynamically at runtime. If/when AOT > compilation is added to JDK, then all stubs and compiled Java will be > susceptible ROP gadgets being found by static analysis and therefore > potentially as vulnerable as C++ code. > > There are a number of places where the VM changes control flow by > rewriting the stack or otherwise. I’ve done some analysis as to how > these could also be used for attacks (which I didn’t want to post here). > These areas can be protected ensuring the pointers to various stubs and > entry points are stored in memory as signed pointers. These changes are > simple to make (they can be reduced to a type change in common code and > a few addition sign/auth calls in the backend), but there a lot of them > and the total code change is fairly large. I’m happy to provide a few > work in progress patches. > > In order to match the security benefits of the Apple Arm64e ABI across > the whole of JDK, then all the changes mentioned above would be > required. This pull request has now been integrated. Changeset: 6fab8a2d Author: Alan Hayward <alan.hayw...@arm.com> Committer: Andrew Dinn <ad...@openjdk.org> URL: https://git.openjdk.java.net/jdk/commit/6fab8a2d6a97dbd2ffceca275716d020cb9f1eea Stats: 1481 lines in 35 files changed: 574 ins; 32 del; 875 mod 8277204: Implement PAC-RET branch protection on Linux/AArch64 Reviewed-by: erikj, ihse, adinn, ngasson ------------- PR: https://git.openjdk.java.net/jdk/pull/6334
